Ultrastructure of cellulose crystallites in flax textile fibres
Résumé
Seven varieties of flax (Linum usitatissimum) fibres were analyzed in order to gain a deeper insight into the morphological features of the crystalline assembly. Different spectroscopic techniques and a chemical bleaching process were used to provide an accurate description of the lateral arrangement of the polysaccharide chains within the fibre cell wall. The flax fibres were analyzed in their natural state and after an extraction treatment of the non-crystalline components such as hemicelluloses, pectins and phenolics. The chemical bleaching process consisted of a Soxhlet extraction in toluene, a sodium chlorite treatment and an alkaline extraction of the residual hemicelluloses. Solid-state 13C nuclear magnetic resonance (NMR) confirmed the sequential removal of the non-cellulosic components from the flax cell wall. Both wide-angle X-ray diffraction (WAXD) and solid-state 13C NMR provided measures of the crystallite thicknesses and overall crystallinities before and after treatment. The existence of non-cellulosic highly ordered paracrystalline domains was also evidenced by proton spin relaxation time calculation. Whereas the overall crystallinity determined by WAXD decreased after treatment, the cellulose crystallinity calculated with the help of the solid-state 13C NMR slightly increased. This is explained by the difference in chemical selectivity between these two techniques and by the paracrystalline state of both hemicelluloses and pectins. Strong adhesion between cellulose crystallites, hemicelluloses and pectins in the fibres was evidenced by low spin–spin relaxation times and by an increase in crystallite thickness after bleaching. A simple model is proposed that describes the rearrangement of the macromolecules during the bleaching process.